The progress in wireless data transmission has more and more brought forth the need to transmit wirelessly not only calls but also different data applications. The circuit-switched connections used conventionally in mobile systems are, however, rather poorly suited for transmitting different burst-type data services, which is why packet-switched applications have also been developed for mobile systems. During the past few years, GSM 2+phase standards, in which the new packet-switched data transmission service GPRS (General Packet Radio Service) is also defined, have been drafted for the European digital GSM (Global System for Mobile communication) mobile network, for instance. GPRS is a packet radio network utilising the GSM network, which endeavours to optimise data packet transmission by means of GPRS protocol layers on the air interface between a mobile station and a GPRS network.
The GSM system comprises several different, typically unidirectional control channels, by means of which the network controls the operation of mobile stations. One such channel is the broadcast control channel BCCH, through which information is transmitted on the different cells of the network, such as identification information of the cell, identification information on the network, frequencies used in the cell, etc. Each base transceiver station BTS transmits information on a cell on a broadcast control channel of its own, to which all mobile stations in the area of the cell listen.
Correspondingly, specific control channels have also been designed for the GPRS system, one of which is the packet broadcast control channel PBCCH, whose task is to transmit system information to all GPRS mobile stations in the cell; just like BCCH does in the GSM network. When a GPRS mobile station is in idle mode, no data transmission resources have been allocated to it and it only listens to the broadcast control channel BCCH and the paging control channel PCH of the cell, or to the packet broadcast control channel PBCCH and the packet paging control channel PPCH, if the cell supports this GPRS control channel. If the cell does not support this PBCCH channel, which will be a very common situation when GPRS networks will be built on top of the GSM network, the GPRS mobile station listens to the GSM broadcast control channel BCCH. The used broadcast control channel, PBCCH or BCCH, is thus defined according to which broadcast control channel is available at each time in the cell of the GPRS mobile station.
A GPRS mobile station should thus support broadcast control channel reception on both the PBCCH channel and the BCCH channel. Then, if the cell uses the PBCCH channel, the network transmits the parameters used in the GPRS network to the mobile stations on said PBCCH channel only.
If a GPRS mobile station is in packet transfer mode, control messages, which in idle mode are only transmitted on the PBCCH or PPCH channels, are transmitted from the network to the mobile station. These messages, which are known as PSI messages, are then transmitted from the network to the mobile station on a PACCH (packet associated control channel) channel which is an internal control channel, i.e. in-band channel, of a traffic channel PDTCH (packet data traffic channel). In such a case, the control messages are transmitted among the data packets that form the actual payload on the same physical channel.
A problem with the arrangement described above is that if the GPRS mobile station cannot correctly receive system information transmitted on the PBCCH channel, it also cannot transmit or receive data packet transmissions according to GPRS. In the near future, when GSM networks will be updated to also support GPRS packet data services, a great risk exists that GPRS mobile stations which came out on the market before the PBCCH channel came to the networks will not work on the PBCCH channels to be built into the networks in the future, because it has not been possible to test them in field conditions. One problem in this is the transmission of said PSI messages to mobile stations on a traffic channel, in which case all mobile stations receive control messages which are intended to be transmitted on packet-switched control channels only. Thus, there also exists a great risk that GPRS mobile stations which came out on the market before the GPRS service was initiated will not work in the GPRS network after the PBCCH channels are taken into use in the new networks. The extremely significant drawback in this is that circuit-switched speech services will not work, either, in such GPRS mobile stations.